1. Field of the Invention
The present invention relates to a method of and an apparatus for manufacturing an optical disc, and more particularly, to a method of and apparatus for manufacturing an optical disc which has a high-density recording capacity and in which a bump formed along a circumference of the optical disc during forming a transparent layer by a spin-coating method can be easily removed.
2. Description of the Related Art
In general, an optical disc is extensively and widely used as an information recording medium in an optical pickup apparatus to which information is recorded in and reproduced from in a non-contact manner. The optical disc has one of two types according to its information recording capacity: a compact disc (CD) and a digital versatile disc (DVD). Especially, an increase in the quantity of the information to be recorded triggers a development in the DVD having a high-density and large capacity.
A spot diameter of a laser beam used in recording/reproducing the information on/from the optical disc is conventionally reduced to increase a recording density of the optical disc. This is based on the principle that the smaller a wavelength λ of the laser beam is and the larger a numerical aperture NA of an objective lens is, the smaller the spot diameter r of the laser beam. The spot diameter r is expressed by the following equation (1):
  r  ∝      λ    NA  
In conclusion, the wavelength of the laser beam must be shorter and the numerical aperture of the objective lens must be larger in order to realize a high-density optical disc. For this reason, a range of lasers from 650 nm red laser to 480 nm blue laser are available as the laser beam and the numerical aperture of the objective lens can be adjusted in the range of 0.5˜1.5 NA.
However, there is a limitation in enlarging the numerical aperture of the objective lens and reducing the wavelength of the laser beam. That is, during reproducing a recorded signal, a degradation W31 of the reproduced signal is in proportion to the cube of the numerical aperture NA3 and a thickness d of the optical disc and is in inverse proportion to the wavelength λ of the laser beam. This is expressed by the following equation (2):
      W    31    ∝            d      ·              NA        3              λ  
From the equation (2), it is noted that if the wavelength λ of the laser beam is shorter and the numerical aperture NA of the objective lens is larger, the spot diameter of the laser beam becomes smaller, but the aberration thereof becomes remarkably increased, thereby degrading characteristics of the reproduced signal. Accordingly, the aberration of the laser beam must be reduced to increase the recording density by adjusting the numerical aperture of the objective lens and the wavelength of the laser beam.
It is required to form a substrate to be thinner so as to increase the recording density of the optical disc and reduce the aberration of the laser beam. Thus, the DVD is made of two pieces each having a 0.6 mm substrate to have the same thickness of 1.2 mm as the CD to achieve a compatibility between the CD and the DVD. Also, the DVD is recently manufactured to have a two-piece or three-piece structure using two or three pieces each having a substrate of 0.3 or 0.4 mm.
Furthermore, in an optical disc shown in FIG. 1, a thickness T of a protective substrate 10 is 1.1 mm, and a thickness d of a transparent layer 15 is 0.1 mm. That is, a total thickness of the optical disc becomes 1.2 mm. Here, a spin-coating method is used for making the 0.1 mm transparent layer 15. According to the spin-coating method, a liquid ultraviolet-hardening resin 13 is applied around a center of the protective substrate 10, which is rotated at a low speed, as shown in FIG. 2. Then, the protective substrate 10 is rotated at a high speed. As a result, the ultraviolet-hardening resin 13 is spread out in a radial direction of the protective substrate 10 due to a centrifugal force, thereby coating the entire protective substrate 10 with the ultraviolet-hardening resin 13. Thereafter, an ultraviolet ray is illuminated on the entire protective substrate 10 to harden the ultraviolet-hardening resin 13 to form the transparent layer 15.
However, high polymer such as the ultraviolet-hardening resin 13 is a viscous-elasticity material that has viscosity and elasticity. Elasticity is a property of the polymer that is transformed due to a stress applied thereon, but it returns to its original state by a removal of the stress according to Hooke's law. Thus, referring to FIG. 1, when the transparent layer 15 is formed by the spin-coating method, a lump 15a of the ultraviolet-hardening resin 13 is formed along the circumference of the optical disc due to the elastic property of the ultraviolet-hardening resin 13 after the protective substrate 10 is rotated at a high speed. The lump 15a of the ultraviolet-hardening resin 13 is hardened as it is and becomes a hilly spot that is called the bump 15a when the ultraviolet ray is illuminated on the transparent layer 1. A central hole 14 is formed in the protective substrate 10 to be used in chucking the optical disc.
Meanwhile, a region in which information is recorded on the optical disc is reduced by a width w of the bump 15a due to an area occupied by the bump 15a. A DVD-RAM format prescribes that a data region in which user data is recorded is 57.9 mm in a diametric direction of the optical disc and a read-out region in which disc data is recorded is 58.5 mm in the diametric direction of the optical disc. This is understood that the read-out region in which the disc data is recorded must be 58.5 mm in the diametric direction of the optical disc which must have at least 117 mm in the diametric direction of the optical disc. However, when a 0.1 mm transparent layer is formed by the spin-coating method as described above, the width w of the bump 15a appears to be larger than 1.5 mm in any case when the thickness of the transparent layer 15 was about 0.1 mm, i.e., 100 μm.
The above experimental result is illustrated in FIG. 3, wherein a horizontal axis denotes the thickness d of the transparent layer 15, and a vertical axis denotes the width w of the bump 15a. FIG. 3 reveals that the width w of the bump 15a is larger than 1.5 mm in most cases if the thickness of the transparent layer 15 is about 100 μm. Therefore, both ends of the optical disc formed with the bumps 15s each having the width w occupies more than 3 mm in the radial direction of the optical disc. For this reason, given that the entire diameter of the protective substrate 10 is n, the information recording capacity of the optical disc is reduced by twice the width w of the bump 15a, and thus, the maximum recording region is substantially reduced to a portion indicated by m in FIG. 1.
Further, given that a height of the bump 15a is h, there is a possibility that a front end of an objective lens (not shown) collides with the bump 15a during recording or reproducing the information on or from the optical disc by an optical pickup apparatus if a distance between the objective lens and the optical disc is shorter than h of the bump 15a which is formed as described.
Therefore, various methods to remove the bump 15a have been devised. For example, the protective substrate 10 is planarized by blowing wind to a circumferential direction of the ultraviolet hardening resin 15 from the center of the protective substrate 10 before the hardening of the ultraviolet hardening resin 15, thereby preventing the occurrence of the bump 15a. However, in this case, since the planarization of the surface of the protective surface 10 is performed before the hardening of the ultraviolet hardening resin 15, a surface of the protective substrate 10 may not become flat but irregular by the wind.
Otherwise, as shown in FIG. 4, after the hardening of the ultraviolet hardening resin 15, the bump 15a formed along the circumference of the protective substrate 10 is trimmed by a trimming tool 25 being rotated by a rotating unit 20. However, this method is also disadvantageous in that it takes a long time to perform a trimming process and the protective substrate 10 is easily polluted by fine dust generated during the trimming process, thus causing the degradation of the recording/reproducing characteristics of the optical disc.